Amphioxus as a Model for Mechanisms in Vertebrate Development

Abstract

For the last two centuries, the cephalochordates, commonly known as lancelets or amphioxus, have been central to investigate
the evolutionary genesis of vertebrates. At first, by classical morphologists fascinated by their odd but at the same time
familiar anatomical traits and later by molecular biologists giving the first insights into their slow evolving nature. The
present data available not only holds amphioxus as an organism of preternatural importance within the tree of life but also
boosts its potential to untangle the molecular basis underlying the vertebrate complexity. This is a walk through the past
and present of the amphioxus field merging morphological and molecular data in linkage with the fossil record and the modern
vertebrates. The resulting picture is drawn together with comparative maps of genome organisation, gastrulation and the origin
of the vertebrate organiser, neurulation and the origin of the neural crest, and shared signalling mechanisms between vertebrates
and amphioxus during development. Special attention is also given to some of the most critical vertebrate novelties and how
the pre‐duplicative amphioxus genetic toolkit might have contributed to set the basis for evolving complexity in the course
of vertebrate evolution.

Key Concepts:

Cephalochordates occupy at present the key phylogenetic position to get insights into the invertebrate–vertebrate transition
and the evolutionary genesis of vertebrates.

Whole‐genome duplications in the lineage leading to modern vertebrates might have established the basis for molecular and
morphological innovation.

The amphioxus genome appears to be the best surrogate available for the ancestral chordate genome regarding the gene content,
gene structure and chromosomal organisation.

The amphioxus prototypical body plan with respect to vertebrates facilitates the comparative analysis and linkage between
invertebrates and vertebrates.

The pre‐duplicative genome of amphioxus offers the possibility of analysing vertebrate‐like gene regulatory networks in a
simpler biological context with a reduced number of molecular players.

Comparative anatomy of fossils, cephalochordates and vertebrates. (a) Haikouella, (b) Juvenile amphioxus and (c) Amnocoete larva of a lamprey (agnathan vertebrate). Although Haikouella fossils remarkably resemble modern cephalochordates the pharyngeal denticles (where indicated) approaches them to vertebrates.
Although in Haikouella and the ammocoete lamprey larva the eyes are paired, the so‐called frontal eye in amphioxus is unique and medially located.
After Mallatt and Chen and Holland et al..

Figure 2.

Schematic representation of macrosynteny between the B. floridae and the human genome. The 17 CLGs identified in the B. floridae genome are represented in (a). The gene content and organisation is in most of the cases clearly conserved between B. floridae and the human genome, allowing the identification of the duplicated genes in their chromosomal context after the genome doublings
(2R) that took place during vertebrate evolution. Such conservation permits to recognise gene losses or chromosomal reorganisations,
amongst other genomic events. The example given in (b) shows the loss of the gene of interest (red) in one of the human chromosomes
(5), the loss of a surrounding gene (blue) in another human chromosome (20) and different arrangements of the paralogous regions,
including a local reorganisation of the surrounding genes (see tandem in pink, orange and blue) in chromosome 12. The ordering
of surrounding genes (pink, orange, blue, yellow and green) is also an example of microsynteny.

Figure 3.

Phylogenetic relationship between vertebrate an invertebrate chordates. (a) Old phyogeny and (b) new phylogeny after Delsuc
and collaborators. Although the first molecular phylogenies located amphioxus as the sister group of vertebrates (a), the
more recent analyses of bigger sets of genomic data place amphioxus in the pivotal position at the root of all chordates.
Such position is, in addition, more consistent with the presence of neural crest‐like (NC‐Like) cells and a mid–hindbrain (MH) boundary in urochordates, still undemonstrated in cephalochordates.

Figure 4.

Comparative representation of the gastrula organiser in vertebrates and amphioxus. (a) Late gastrula of amphibians and (b)
late gastrula of amphioxus. In both cases the vegetal pole is the initial point of invagination, where the blastopore will
remain open. Such opening generates the ventral and dorsal blastopore lips, the latter containing the organiser region (Speman's
organiser in amphibians). Despite differences in the mode of gastrulation, amphibians and amphioxus share an organiser region
with a same embryonic origin and a same genetic code, suggesting a common role in A/P and D/V patterning. DL: dorsal lip;
VL: ventral lip; B: blastopore. Based on references Yu et al. and Holland and Holland .

Figure 5.

Commonalities of Hox regulation between vertebrates and amphioxus. The model represents the collinear‐nested expression of
different Hox genes (HN) delimited by signalling pathways such as those of the retinoic acid (RA) (pink), Cdx (sky blue) and Fgfs (arrows), in both vertebrates (a) and amphioxus (b). Graded pink represents the RA gradient, with a rostral limit coincident with the most anterior Hox expression and a caudal limit partially overlapped with
Cdx expression. Within the presomitic mesoderm (PSM) opposing RA gradients and Fgf/Wnt gradients (grape colour arrows) meet to generate the determination front, from where new somites will
bud, as it is known in vertebrates. In the anterior region, Fgf signalling represses the RA front generating a Hox free region devoid of segmentation. CV: cerebral vesicle; S: somite; PEN: Prosencephalon; MEN: Mesencephalon;
REN: Rombencephalon; R: Rombomere. Based on references Schubert et al., Dequeant and Pourquie , Garcia‐Fernàndez et al. and H. Escrivà (personal communication).

Figure 6.

Territorial differences and common molecular players during the formation of the neural tube in vertebrates and amphioxus.
(a) Cross‐section of a consensus vertebrate in neurula stage and (b) cross‐section of an amphioxus neurula. The major difference
lies on the neural crest territory at the edges of the neural plate (light yellow). In vertebrates (a) the neural crest territory
is separating the nonneural (E) from the neural ectoderm (NP), whereas in amphioxus (b) the nonneural ectoderm (E) simply closes over the neural ectoderm (NP) (light yellow territory is represented where it conceptually should be located). Although the molecular machinery to set
the neural crest borders is common between amphioxus and vertebrates, only one neural crest specifier, Snail, is equally restricted
in the equivalent region in amphioxus. The notochord underlying the neural plate has been removed for clarity. E: epidermis;
PM: Paraxial Mesoderm; NP: neural plate; NT: neural tube. Based on references Holland and Yu et al..

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